The invention relates to α/β-sialon-based materials. In particular, the invention relates to α/β-sialon-based materials having improved sintering activity and high edge strength of the sintered molded articles produced from the materials.
Sintered molded articles composed of α/β-sialon, particularly for use as a cutting tool, for example a cutting insert, are known from the state of the art. The mixture of α-sialon and β-sialon allows the production of sintered molded articles that demonstrate great hardness on the basis of the grainy α-sialon, on the one hand. On the other hand, however, the sintered molded articles also possess good impact resistance on the basis of the needle-shaped β-sialon grains.
For use as a cutting tool, it is required, in addition to sufficient hardness and impact resistance, that the material is also temperature-resistant, because very great heating of the cutting tool can occur locally, particularly when cutting gray cast iron or nickel-based alloys (superalloys), and here, in particular, using a continuous cut. However, many oxidic sintering aids that are present essentially in the glass phase after sintering of the material have only comparatively slight temperature resistance and low heat conductivity. Therefore, overheating quickly occurs locally, and the glass phase softens. Furthermore, oxidation of other components can occur due to the temperature effect, and this can lead to premature failure of the cutting tool, particularly due to spalling that proceeds from the cutting edge, suddenly increased abrasive wear or oxidation of the other components (Si3N4, sialon or TiN).
If the cutting tool is used in an interrupted cut, the temperatures are lower than in a continuous cut. The cutting insert is not in constant contact with the workpiece, so that it can cool off a little, again and again. However, in an interrupted cut, resistance to cracking and edge stability should be comparatively great, since the mechanical stress is much greater than in a continuous cut.
α/β-sialons having final densities greater than 99% of the theoretical final density are currently either sintered at temperatures above 1750° C. and simultaneously an elevated nitrogen partial pressure or compacted without pressure and with large amounts of oxidic additives.
Sintering under elevated nitrogen partial pressure requires a closed furnace system and, in comparison with sialons sintered without pressure, more energy and gas (nitrogen). For these reasons, pressure-free compaction, for example under nitrogen flow at temperatures below 1750° C., is fundamentally more efficient than gas-pressure sintering.
However, since the type and amount of the oxidic additives determine the high-temperature properties of the sialon ceramic, increased wear is observed during high-temperature applications, such as, for example, when cutting gray cast iron, in the case of sialons compacted without pressure as compared with gas-pressure-sintered sialons.